Zinc oxide (hereinafter referred to as ZnO) based semiconductor crystals as a novel crystalline material have attracted considerable attention as an alternative to III-V group nitride semiconductor crystal used in blue-light emitting devices, ultraviolet light emitting devices or the like.
Here, a ZnO-based semiconductor crystal includes a non-doped ZnO, ZnO-based mixed crystal such as zinc magnesium oxide (ZnMgO), zinc cadmium oxide (ZnCdO), ZnO or a ZnO-based mixed crystal that are doped with gallium (Ga), nitrogen (N) or the like and show a specific electronic conductivity.
In order to realize blue-light emitting devices or ultraviolet light emitting devices using the ZnO-based semiconductor crystal, the ZnO-based semiconductor crystal, the ZnO-based semiconductor crystal is required to have an excellent surface flatness and excellent crystallinity.
For example, techniques described in Non Patent References 1 to 3 have been conventionally proposed so as to satisfy the above-described requirements.
Non-Patent Reference 1 describes a growth of a non-doped ZnO semiconductor crystal (hereinafter referred to as ZnO crystal) at a very high crystal growth temperature (substrate temperature) using a laser molecular beam epitaxy (laser MBE) apparatus. Specifically, a ZnO crystal of excellent surface flatness and crytsallinity is realized by crystal growth on a substrate (in Non-Patent Reference 1, a scandium aluminum magnesium oxide substrate) heated at 800° C. by ablation of sintered ZnO as a raw material using a krypton fluoride (KrF) excimer laser.
On the other hand, a molecular beam epitaxy (MBE) method is known as an alternative method for growing a ZnO crystal of high quality. For example, Non-Patent Reference 2 describes a general method of growing a ZnO crystal by the MBE method. In the method described in Non-Patent Reference 2, by heating a Knudsen cell filled with solid zinc (Zn), the solid Zn is partially evaporated and is introduced to the surface of a substrate (in Non-Patent Reference 2, a sapphire substrate). At the same time, radicalized oxygen (O radical) gas is introduced to the surface of the substrate from a different side. Thus, ZnO crystal is grown by a reaction between Zn and O radical on the surface of the substrate. In the MBE method, it is possible to reduce the amount of impurities in the grown Zn crystal to an extremely low level by using highly pure solid Zn and O2 gas as raw materials, and by maintaining the atmosphere of crystal growth in a high vacuum. In the MBE method, ZnO crystals are generally grown at a crystal growth temperature of about 600 to 700° C. (600° C. in Non-Patent Reference 2).
A reactive ion cluster beam (R-ICB) method is known as an alternative method of growing a ZnO crystal. Non-Patent Reference 3 describes a general method of growing a ZnO crystal in accordance with the R-ICB method. At the method described in Non-Patent Reference 3, Zn clusters (a state at which a plurality of Zn atoms are bonded by Van der Waals force) are formed by heating a solid Zn filled in a crucible and partially evaporating the solid Zn. The Zn clusters are partially or totally ionized (Zn+) and introduced to a surface of a substrate (in Patent Reference 3, a glass substrate or a sapphire substrate). At the same time, O2 gas is supplied through the passage for ionizing the Zn clusters. The O2 gas is partially ionized (O−) and is introduced to the surface of the substrate. Thus, a ZnO crystal is grown by a reaction of a Zn(Zn+) cluster and O(O−) on the surface of the substrate. In the R-ICB method, Zn clusters and O are ionized and introduced to the surface of the substrate, thereby enhancing their surface migration effect. As a result, it is possible to grow a ZnO crystal having a relatively good crystallinity at a low crystal growth temperature.    Non-Patent Reference 1: A. Tsukazaki et al., “Layer-by-layer growth of high-optical-quality ZnO film on atomically smooth and lattice relaxed ZnO buffer layer” Appl. Phys. Lett., 83 (2003), pp. 2784-2786.    Non-Patent Reference 2: K. Nakahara et al., “Growth of Undoped ZnO Films with Improved Electrical Properties by Radical Source Molecular Beam Epitaxy” Jpn. J. Appl. Phys., 40 (2001), pp. 250-254.    Non-Patent Reference 3: K. Matsubara et al., “PROPERTIES OF ZnO FILMS PREPARED BY REACTIVE IONIZED CLUSTER BEAM DEPOSITION” Surface Science, 86 (1979), pp. 290-299.